In atomic layer deposition (ALD), a substrate placed in a reaction space is subjected to alternating pulses of at least two different reactants suitable for producing a desired thin film on the substrate. When the substrate is exposed to a pulse of the first reactant, a monolayer of the first reactant is chemisorbed on the surface of the substrate until all of the available surface sites are occupied with an adsorbed portion of a reactant molecule and saturation occurs. The surface sites may be occupied by the whole of or by a fragment of a vapor-phase reactant molecule, such as when a metal chloride molecule chemisorbs on a surface site that has a hydroxyl group. For example, TaCl5 may chemisorb as a TaCl4 fragment, with HCl released as a gaseous byproduct. The reaction is chemically self-limiting because gaseous first reactant will not adsorb or react with the portion of the first reactant that has already been adsorbed on the substrate surface. The excess of the reactant is then removed by purging the reaction space with an inert gas and/or evacuating the reaction space.
Subsequently, the substrate is exposed to a pulse of the second reactant, which chemically reacts with the adsorbed portion of the first reactant molecules until the reaction is complete and the surface is covered with a monolayer of the chemisorbed portion of the second reactant. Fragments of the second reactant will be adsorbed under some reaction conditions. For example, when water is used as the second reactant it may leave a fragment of H2O, such as hydroxyl groups (—OH) on the surface. Similarly, when ammonia (NH3) is used as the second reactant it may leave fragments such as NH or NH2 groups on the surface. Reaction conditions such as temperature and pressure are chosen to ensure that physisorption of more than one monolayer of the first or second reactant (or portions thereof) on the substrate cannot occur. In this way the growth of the film proceeds atomic layer by atomic layer.
In the art of atomic layer deposition, the temperature of the substrate is not considered to be very critical because the growth rate of the film is not dependent on the temperature of the substrate but rather on the sequential exposure to the different reactant pulses. In fact, relative temperature independence of the process is a primary advantage of ALD, leading to perfect step-wise coverage despite temperature non-uniformities across large substrates. The temperature is preferably high enough to prevent condensation of the reactants on the substrate and to allow the reaction to proceed at a sufficiently high rate. On the other hand, the substrate temperature preferably remains below the limit where thermal decomposition of the individual reactants occurs. For many combinations of reactants, such as metal halides and water, the reaction is able to proceed at temperatures as low as room temperature and as high as the thermal decomposition temperature limit for the reactants. Thus, a wide temperature window for atomic layer deposition is available.
Accordingly, the temperature of the wall of the reactor is not considered to be an important parameter for ALD. Both hot wall and cold wall designs have been used. In reactors with automated substrate transfer, cold wall designs are typically used. A cold wall reactor design is described in U.S. Pat. No. 5,879,465 to Genus Inc. The reactor described comprises a heater, adapted for heating a substrate supported on a support pedestal, and cooling lines for passing coolant through a portion of the body. This design results in a lower region of the reactor that is hot and an upper region of the reactor that is cool.
In conventional chemical vapor deposition (CVD), a cold-wall design is an advantage. In such a design, where only the substrate, placed on a substrate support, is heated, deposition on the cold wall is prevented. This reduces the required cleaning frequency of the system. However, in contrast, cold regions in the wall of an ALD reactor are particularly harmful for the process for a variety of reasons. First of all, increased adsorption or even condensation of the reactants on the cold region of the wall can occur. Physisorbed or condensed material sticks well to a wall at low temperature and thus may not be effectively removed from the reaction space during the purge between the two reactant pulses. This can result in extra consumption of material and accelerated contamination of the reactor wall. Again, this is contrary to conventional CVD where a reduced wall temperature results in reduced contamination of the wall. Furthermore, when the reaction space above the substrate is on the average at a much lower temperature than the substrate itself, more gas is required for purging the reaction space between the reactant pulses because of the increased gas density at low temperature.
Hot wall batch reactors for ALD are known in the art and they avoid the above-noted disadvantages of cold wall ALD reactors. However, heating of the substrates in hot wall batch reactors occurs indirectly and proceeds very slowly at the relatively moderate temperatures used for ALD. Additionally, the loading of substrates in these hot-wall batch reactors is difficult to automate, making them less suitable for production purposes.
It is an object of the present invention to provide a reactor for ALD, comprising automated substrate transfer into and out of the reactor, that avoids the above described disadvantages and provides an improved control of the ALD process. It is a further object of the invention to provide a method for atomic layer deposition in which harmful reactions on the reaction chamber walls are prevented. It is another object of the invention to provide a method for atomic layer deposition that provides a lower deposition rate on the walls of the reaction chamber than on the substrate, while avoiding harmful reactions on the walls that disturb ALD growth on the substrate.